When the Bible says a thing, no one should deny it. That the Lord opened Lydia’s heart is the plain statement of the text itself, and should not be questioned (Acts 16:14). But the text does not say “how” he opened her heart.
The “how” should not be assumed if there is any way to learn what it was. Several years later, Paul, who preached to Lydia, said the gospel was the power of God unto salvation. (Rom. 1:16.) The word “heart,” when a change is under consideration, means the mind, not the physical heart (Heb. 8:10 compared with 10:16).
To open the heart means to enlighten the mind. If to open the heart means to enlighten the mind, then her heart was opened when she heard Paul’s preaching. David said: “The opening of thy words giveth light; it giveth nnderstanding unto the simple” (Ps. 119: 130). Besides this, Paul was sent to “open people’s eyes” (Acts 26:16-18).
Opening the eyes also means to give enlightenment (Eph. 1:18); hence, it means the same as opening the heart. Since Paul was sent to do the very work in question and was there preaching the gospel to Lydia, the only reasonable conclusion is that God opened her heart through Paul’s preaching. Being thus enlightened, she was ready to obey the commands Paul gave her. Her faith came through hearing the word. (Rom. 10: 17).
John T. Hinds, Gospel Advocate, 1935